1. Field of the Invention
The present invention relates to an alloy for a fuse element of a high temperature fuse, the high temperature fuse and a method of indicating a high temperature condition.
2. Prior Art
In devices which operate in an atmosphere of a high temperature such as electric furnaces, after burners and manifold reactors for automobiles, overheating of the device by exceeding an upper temperature limited of approximately 1100.degree. C can cause structural damage such as oxidation of wall surfaces or even the melting of various parts of the device. To indicate when the device reaches an excessive temperature which may result in overheating, it has been suggested to utilize an indicating device in an electrical circuit which has a thermo fuse or switch. When the fuse element of the fuse melts or breaks due to beginning to melt, the indicating device such as a lamp is energized to indicate that a predetermined temperature has been reached. By utilizing the signal provided by the indicating device, corrective action can be taken to prevent any detrimental effects on the equipment or device due to overheating. Since pure copper has a melting point of 1083.degree. C, it has been suggested to use copper for a fuse element in a fuse to indicate reaching a temperature in a range of 1000.degree. to 1100.degree. C. While pure copper has the desirable melting point at a fixed temperature, it has a very low resistance to oxidation when exposed to an atmosphere at high temperatures. Thus, pure copper is unsatisfactory for use as a fuse element under these conditions.
Since pure copper has been found unsatisfactory as a fuse element, alloys based on nickel, iron, cobalt and the like which alloys have a high oxidation resistance and melt within a temperature range of 1000.degree. to 1100.degree. C have been suggested. However, due to the constituents used in these alloys, the particular alloys do not melt at a fixed temperature or in other words the alloys will not change from a solid phase to a liquid phase or molten state at a fixed temperature in the manner that pure copper does.
As one of the particular alloys is heated through a range of temperatures, a portion of the alloy will begin to be changed to a liquid phase at a given temperature with the amount of liquid phase increasing as the temperature of the alloy is raised above the given temperature until the alloy reaches a temperature at which the alloy is completely molten. The range between the lowest temperature at which the first liquid phase appears and the temperature at which all of the alloy is in a molten state is hereafter referred to as the range of melting temperatures for the alloy and the range of melting temperatures becomes wider as the amount of alloying constituents in the alloy is increased. The strength of these alloys decreases as the volume of liquid phase therein increases over a wide range of melting temperatures so that the strength of these alloys is gradually decreased over a wide range of melting temperatures. Since the melting and breaking temperature of these proposed alloys is not fixed in a narrow range of temperatures, they are unsuitable for use in a fuse element. For satisfactory use, the alloy or material used in the fuse element must have a good oxidation resistance and have a melting and breaking temperature at a fixed temperature which is defined as a temperature in a very narrow range of temperatures such as within a range of 10.degree..